1. Field of the Invention
This invention relates to a wireless communication system in which switching is performed among terminals or between terminals and a public line in wireless fashion, as well as to a method of controlling this system.
2. Description of the Related Art
Wireless communication is rapidly becoming more widespread and is finding use in a variety of fields. Telephone exchanges are no exception, and systems have been proposed in which communication between a main unit having a switching function and wireless telephones dedicated to the system (hereinafter referred to as wireless telephones) is performed in wireless fashion.
A wireless telephone exchange according to the prior art will now be described.
 less than System Architecture greater than 
In a conventional wireless communication system, wireless transmission for low-power analog cordless telephones is employed for wireless communication between extension terminals and the main unit. Specifically, the modulation adopted is frequency modulation, two control channels and 87 voice channels can be used and the communication that is possible is 1:1 (point-to-point). In order that a wireless extension terminal may communicate with the main unit, a connecting device for the wireless extension terminal is required.
At the start of communication, first the voice channel to be used is decided by the control channel, a transition is then made to the voice channel and communication is continued using this channel.
Each component of the conventional wireless communication system and the basic operation thereof will now be described.
 less than Construction of Main Unit greater than 
FIG. 34 is a block diagram showing the architecture of a conventional wireless communication system and the construction of a main unit thereof. A main unit J1, which is the principal component of an exchange system, accommodates a plurality of outside lines and a plurality of terminals and is adapted to switch calls among these. In order that a wireless terminal (referred to as a wireless telephone below) J3 to which a connecting device J2 is connected in point-to-point fashion can be accommodated in the system, the connecting device J2 is placed under the control of the main unit J1 and wirelessly controls the wireless terminal to establish a wireless transmission link. The wireless telephone J3 is a terminal for communicating, via the connecting device J2, with an outside line accommodated in the main unit J1 and with extensions.
The main unit J1 accommodates a PSTN (an existing public telephone network) J4, which is one outside line network, a PSTN line J5, which is an outside line from the PSTN J4, and an SLT (single line telephone) J6, which is one terminal connected to the main unit J1.
The internal construction of the main unit J1 will now be described.
As shown in FIG. 34, a CPU (J101) lies at the heart of the main unit J1 and supervises overall control of the main unit, inclusive of control of switching. The control program of the CPU (J101) is stored in the ROM J102. A RAM (J103) stores various data for control of the CPU (J101) and provides a work area for various computations.
A communication channel unit J104 switches calls (by time division switching) under the control of the CPU (J101). A PSTN line i/f (J105) performs PSTN line control, such as incoming call detection, selection-signal transmission and DC loop closure, under the control of the CPU (J101), in order to accommodate the PSTN line J5. An SLT i/f (J106) performs current feed, loop detection, selection-signal reception and call-signal transmission under the control of the CPU (J101) in order to accommodate the SLT (J6).
A telephone unit J107 has a handset, dialing keys, a speech circuit and a display, etc., for functioning as an extension multi-function telephone under the control of the CPU (J101) when the supply of power to the main unit is normal, and for functioning as an SLT in the event of a power failure. A tone transmission circuit J108 transmits various tones, such as a PB signal, dial tone and incoming call tone. A connecting device i/f (J109) sends and receives a voice signal and control signal to and from the connecting device J1 under the control of the CPU (J101) in order to accommodate the connecting device J2.
 less than Construction of Connecting Device greater than 
FIG. 35 is a block diagram showing the construction of the connecting device J2 in the conventional wireless communication system. As shown in FIG. 35, the connecting device J2 has a CPU (J201) for controlling the overall connecting device, inclusive of control of the communication channel and wireless portions. A ROM (J202) stores the control program of the CPU (J201), and an EEPROM (J203) stores the call code (system ID) of the system. Furthermore, a RAM (J204) stores various data for control of the CPU (J201) and provides a work area for various computations.
A main unit i/f (J205) sends and receives a voice signal and control signal to and from the main unit J1 under the control of the CPU (J201). A PCM codec J206, which is under the control of the CPU (J201), converts a PCM-coded speech signal from the main unit i/f (J205) to an analog voice signal, transmits the analog voice signal to a voice processing LSI (J207), converts an analog voice signal from the voice processing LSI (J207) to a PCM code and transmits the PCM code to the main unit i/f (J205).
Under the control of the CPU (J201), the voice processing LSI (J207) receives a demodulated signal from a wireless unit J208, described below. If the received signal is control data, the voice processing LSI performs an A/D conversion and sends the converted data to the CPU (J201). If the received signal is a voice signal, the voice processing LSI executes processing such as descrambling and decompression and outputs the processed data to the PCM codec (J206). At the same time, the voice processing LSI subjects control data transmitted from the CPU (J201) to a D/A conversion, transmits the analog data to the wireless unit J208, subjects a voice signal from the PCM codec (J206) to scrambling such as scrambling and compression and transmits the processed signal to the wireless unit J208.
Under the control of the CPU (J201), the wireless unit J208 executes processing in such a manner that the control data and voice signal from the voice processing LSI (J207) can be modulated and wirelessly transmitted, transmits the processed signals to the wireless telephone J3, demodulates a signal received from the wireless telephone J3, extracts control data and a voice signal and transmits the results to the voice processing LSI (J207).
 less than Construction of Wireless Telephone greater than 
FIG. 36 is a block diagram showing the construction of the wireless telephone J3 accommodated in the conventional wireless communication system. As shown in FIG. 36, the wireless telephone J3 has a CPU (J301) for controlling the wireless telephone J3, inclusive of wireless control and call control. A ROM (J302) stores the control program of the CPU (J301), and an EEPROM (J303) stores the call code (system ID) of the system and a sub-ID of the wireless telephone J3. A RAM (J304) stores various data for control of the CPU (J301) and provides a work area for various computations.
A communication channel unit J305 inputs and outputs speech signals to and from a handset J308, microphone J309 and speaker J310 under the control of the CPU (J301). Under the control of the CPU (J301), a voice processing LSI (J306) receives a demodulated signal from a wireless unit J307. If the received signal is control data, the voice processing LSI performs an A/D conversion and sends the converted data to the CPU (J301). If the received signal is a voice signal, the voice processing LSI executes processing such as descrambling and decompression and outputs the processed data to the communication channel unit J305. The voice processing LSI subjects control data transmitted from the CPU (J301) to a D/A conversion, transmits the analog data to the wireless unit J307, subjects a voice signal from the communication channel unit J305 to processing such as scrambling and compression and transmits the processed signal to the wireless unit J307.
Under the control of the CPU (J301), the wireless unit J307 executes processing in such a manner that the control data and voice signal from the voice processing LSI (J306) can be modulated and wirelessly transmitted, and transmits the processed signals to the wireless connection unit J2. At the same time, the wireless unit J307 demodulates a signal wirelessly received from the wireless connection unit J2, extracts control data and a voice signal and transmits the results to the voice processing LSI (J306).
The handset J308 inputs and outputs voice signals in order that a system user may communicate, the microphone J309 collects and enters voice signals and the speaker J310 emanates a voice signal. A dialed number entered from a key matrix J311, the status of outside lines, etc., are displayed on a display unit J312. The key matrix J311 comprises dialing keys (not shown) for entering telephone numbers, outside line keys, a hold key and function keys such as a speaker key.
 less than Operation of Conventional Wireless Communication System greater than 
The basic operation of the conventional wireless communication system will now be described.
FIG. 37 illustrates the operating sequence of the conventional wireless communication system. When a request for originating a call is made at the wireless telephone J3, the latter sends the connecting device J2 a connection notification signal on a predetermined wireless control channel (sequence J401). Upon receiving the connection notification signal, the connecting device J2 checks the status of the wireless speech channels and, if a usable speech channel exists, sends a connection answer signal to the wireless telephone J3 (sequence J402).
Upon receiving the connection answer signal, the wireless telephone J3 changes over the frequency used from the wireless control channel to the wireless speech channel and transmits a channel-shift notification signal to the connecting device J2 (J403). Signals are sent and received on the speech channel from this point onward.
Upon receiving the shift notification signal, the connecting device J2 acknowledges the transition to the speech channel and transmits a channel-shift answer signal to the wireless telephone J3 (J404). The connecting device J2 then sends the main unit J1 notification of line connection (J405), after which a transition is made to the speech communication state.
If the wireless telephone J3 receives the channel-shift answer signal in sequence J404 and confirms establishment of a wireless link, the wireless telephone J3 sends the connecting device J2 an outside-line outgoing call signal (J406). Upon receiving the outside-line outgoing call signal, the connecting device J2 originates an outside-line call in the main unit 1 (J407). When there is a voice connection request from the main unit J1 to the connecting device J2 (J408) and from the connecting device J2 to the wireless telephone J3 (J409), a dial tone is transmitted (J410). Accordingly, it is possible to dial for making an outgoing call (J411). From this point onward, the outgoing call to the outside line is implemented. When the called party answers, a transition is made to speech communication (J412).
Through the procedure described above, a wireless telephone is capable of placing a telephone call via a public switched telephone line. With regard to an incoming call as well, a wireless speech channel is acquired and a telephone call can be initiated and continued through a procedure similar to that described above.
Similarly, when an extension call is placed, the call between extensions is implemented via the main unit.
There is also a system in which wireless telephones can perform direct extension communication with each other without the intermediary of the main unit. In the case of such a system, a wireless telephone on the originating side itself acquires an idle wireless link and notifies a wireless telephone on the called side. The notified wireless telephone performs extension communication by effecting a transition to the wireless link of which it has been notified.
In this conventional wireless exchange system, however, narrow-band frequency modulation is used as the method of wireless modulation. As a result, the transmission rate is limited and, hence, the audio from a plurality of wireless telephones cannot be multiplexed using a single carrier frequency. Accordingly, the connecting device and the wireless telephone must be used in a one-to-one (point-to-point) relationship. If the number of wireless telephones accommodated increases, the number of connecting devices must be increased. There is also a limit upon the number of connecting devices that can be accommodated by the main unit and upon the number of wireless telephones that can be accommodated.
In the case where extensions communicate via the main unit, the main unit is used as a relay and is subjected to a load. In the case where wireless telephones place extension calls to each other directly, the wireless telephones acquire the wireless channel. This means that the main unit cannot determine whether this wireless channel is currently being used or is incapable of being used because of disturbing waves.
Further, in the case where wireless telephones place extension calls to each other directly, the status of use of the wireless telephones and the status of use of the wireless channels cannot be managed by the main unit. Furthermore, since a wireless telephone J3 communicates using the wireless channel which it itself acquired, control information cannot be received from the main unit.
An object of the present invention is to provide a wireless communication system in which selection of communication means conforming to a communication request and status of communication is possible.
Another object of the present invention is to reduce the load upon the main unit in communication between extensions.
A further object of the present invention is to make it possible for the main unit to implement centralized administration of wireless terminals and wireless channels used in a wireless communication system.
Yet another object of the present invention is to provide a wireless communication system in which control information can be sent and received between the main unit and wireless terminals even when the wireless terminals communication with each other directly.
A further object of the present invention is to provide a wireless communication system that is easy to operate.
A further object of the present invention is to provide a wireless communication system that is highly flexible.
A further object of the present invention is to provide a wireless communication system featuring excellent communication security and resistance to noise.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.